the subject of the present invention is a method and a plant for purifying wastewater, especially municipal or industrial wastewater, comprising an additional sludge treatment by ozonation in order to reduce the amount of sludge produced by a biological treatment and to increase the flows passing through the sludge clarification system.
The present invention also proposes reducing, on the one hand, the cost of treating excess sludge and, on the other hand, the costs of investing in the water-sludge separation system.
It is known that wastewater treatment largely makes use of activated sludge methods which are based on the principle of the natural flocculation of bacterial populations brought into contact with the water to be purified. Thus a mixture called a mixed liquor is obtained, which is placed under the aeration conditions needed for the biochemical transformation of the organic carbon or nitrogen matter which are contained in the water to be treated.
This involves high-performance methods which are widely used, but the hydraulic treatment capacity of which is frequently limited because of biological problems which negatively affect the settleability of the flocculated sludge (a proliferation of this sludge is produced) and, consequently, reduce the permissible flow rate during clarification which is the final and key step of the treatment.
Moreover, it is known that the biological treatment methods have the drawback of producing large amounts of excess sludge, the final destination of which raises problems with regard to protecting the environment and the removal cost of which is increasingly prohibitive.
In order to solve these problems, various solutions have been proposed. By way of example, the use of chlorine or of ozone to reduce the proliferation of filamentous organisms will be mentioned. By way of an example of this solution, mention may be made of the publication xe2x80x9cDomestic and Industrial Wastewater Treatment with Ozonated Activated Sludgexe2x80x9d by H-van Leeuwen in xe2x80x9cOzone Science and Engineeringxe2x80x9d, Vol. 10, pp 291-308 (1988). With regard to the problem of reducing the production of sludge, reference may be made to EP-A-0 645 347 which describes a method in which the sludge is treated with ozone, coupled with an acidic pretreatment of the sludge, so as to reduce the production of excess sludge. A variant of this known treatment is described in FR-A-2 766 813 which describes a method in which the ozonation treatment is coupled with a mechanical treatment of the sludge.
It will be noted that implementing these known methods enables a marked improvement in the settleability of the sludge (between 20 and 50%) to be obtained.
The possibility of obtaining sludge having the appearance of granules or of granular sludge is also known.
In an anaerobic medium, the method of obtaining these granules consists in adding xe2x80x9cnucleixe2x80x9d or even micro-carriers and in applying high ascending speeds to the water to be treated in the anaerobic reactor. An application example of this method is given in the publication xe2x80x9cGranular Sludge Formation in the Anaerobic Expanded Micro-Carrier Processxe2x80x9d in xe2x80x9cWater Science Technologyxe2x80x9d Vol. 23, pp 1167-1177.
In an aerobic medium, a first application example of obtaining aerobic granular sludge is given in the publication xe2x80x9cAerobic Granulation in a Sequencing Batch Reactorxe2x80x9d by J. J. Beun et al., in xe2x80x9cWater Researchxe2x80x9d Vol. 33, No. 10 pp 2283-2290, 1998. The technology used is of the SBR (Sequencing Batch Reactor) type and the mode of obtaining granular sludge is very dependent on the operating conditions of the plant and necessarily passes through a first step leading to the formation of filamentous granules, with specific moulds of the Fungi type.
Starting from this prior art, the object of the present invention is to solve the problem consisting in directly obtaining flocs having the appearance of granules or of granular sludge, making it possible to improve the settleability of the sludge, and this in an aerobic medium without specific seeding.
According to a first aspect, the present invention relates to a method of purifying wastewater charged with organic matter which comprises a step of biologically treating the water, during which the organic matter contained in the water is degraded by micro-organisms thereby producing sludge, and a water-sludge separation step, the sludge coming from the separation step being recycled in the biological treatment step, this method being characterized in that it further comprises a step of degrading the sludge, coming either from the biological treatment step or from the separation step, during which the sludge is brought into contact with an ozonated gas under conditions making it possible to obtain a floc consisting of granules whose mean size is greater than 200 microns, the volatile matter content is between 50 and 65%, the thickening factor of this granular sludge, after 30 minutes of settling, always being greater than 4, the conditions for obtaining the said granular sludge consisting in:
treating between 0.1 and 2 times the mass of sludge present in the biological treatment per day and preferably between 0.7 and 1.5 and,
applying an ozone dose of between 3 and 100 grams of ozone per kilogram of treated suspended matter (SM), preferably between 4 and 10 grams of ozone per kilo of treated SM.
According to the present invention, the ozonation step may be implemented continuously or batchwise.
An explanation of the phenomena discovered by the present proprietor is given below. The action of the ozone causes stress in the bacterial metabolism, characterized by the solubilization of organic and inorganic matter and a substantial modification of the bacterial metabolism. The enzymatic reaction in relation to stress is called maintenance energy. One of the visible causes is the regrouping of the floc in a new granular structure, that is to say physical self-protection of the bacterial fauna against stress.
According to a second feature of the invention, the bacteria will use the substrate from the treated water no longer to reproduce but to withstand the effects of stress by increasing the maintenance energy.
According to a second aspect, the subject of the present invention is a water treatment plant implementing the method defined above.
This plant comprises at least one biological treatment reactor receiving the raw water charged with organic matter which is degraded by micro-organisms thereby producing sludge and a solid/liquid separator which receives the wastewater coming from the biological reactor and which separates the water from the sludge, a recycling loop enabling at least part of the sludge thus separated to be returned to the biological reactor. This plant is characterized in that it comprises a means for degrading the sludge, which collects part of the sludge coming either from the biological reactor or from the separator and subjects this sludge to an ozonation treatment, the sludge thus treated then being recycled in the biological reactor.